1. Field of the Invention
This invention in general relates to endoscopes and in particular to endoscopic systems which employ gradient index elements.
2. Description of the Prior Art
The term endoscope refers to a broad class of optical instruments used in medical and industrial applications, wherein they are known as borescopes, to visualize what would otherwise be impossible to see with the unaided eye. In the medical field, they are passed through natural or surgically created openings in the body to examine interior organs or cavities and to perform surgical and biopsy procedures. Their less well known industrial applications include their widespread use for inspecting the interiors of a variety of devices including engine cylinders, pipes, boilers, and the like.
Because they are used to visualize the interior of objects through small openings, endoscopes are inherently long in relation to their diameters. In their simplest, and undoubtedly, oldest form they are nothing more than a rigid tube open at both ends with lighting for the object provided either at the distal end (near the object) or shone down the tube from the proximal end (near the observer).
More complex versions, which are improvements on the simple tube, are small telescopes or periscopes that are made up of a series of elements usually including an objective lens which forms an image near the distal end. The objective is then followed by a series of relay and field lenses which transfer this image to a location near the proximal end where it is observed through an ocular or falls on a videocamera sensor and is viewed on a monitor. Lighting is again provided by tiny bulbs near the distal end or piped from the proximal end to the object with fiber optics or other illumination schemes. Where something other than direct line viewing is required, line-of-sight prisms are used at the distal end to change the direction of view away from the longitudinal axis of the instrument.
More recently, graded or gradient index optical elements have been used in endoscopes to perform objective and relay tasks where small caliber (2 mm or less) instruments are required to examine or operate in small spaces such knee or even smaller joints such as the wrist, ankle, or temporal-mandibular joint. For these small caliber applications, graded index optical elements represent a practical and economic solution compared with their homogeneous counterparts. With homogeneous materials of necessarily variable geometry, both initial fabrication at the required scale and their subsequent assembly in proper alignment as an instrument is difficult and expensive. In contrast, graded index elements, which are made of inhomogeneous materials, have a regular right circular cylindrical geometry which permits them to be easily aligned as in-line monolithic assemblies. Thus, for small caliber applications, graded index materials are superior because of geometric advantages.
In addition to scale and ease of assembly, endoscopes of gradient index elements can be cemented together to provide slender systems that are sealed throughout their length to eliminate the possibility for entry into the optical path of dust, moisture, or other foreign matter which could have deleterious effects on performance or function.
Graded index optical elements of the type used in endoscopic systems are commercially available with a variety of properties that are well understood by those practicing in the optical arts. Computer programs are widely available for computing designs employing such elements exclusively and in hybrid systems in combination with homogeneous lenses.
Graded index elements of cylindrical symmetry, often referred to as GRIN rods, have an approximately quadratic variation of index of refraction or, in any case, a radial variation in index that is describable mathematically as a polynomial of even ordered terms of the radius with the coefficients related to index variation. Such elements are available in the United States from NSG America under the tradename SELFOC.RTM.. In these rods, or fibers as they are sometimes called, a meridional ray follows a sinusoidal or helicoid (spiral) path as it travels through the element. The length over which the sinusoid or helicoid goes through one complete cycle is referred to as a period and is easily calculable from literature supplied by manufacturers or available in scientific publications. If the ends of such a rod are polished flat and perpendicular to its axis, then an image of an object at a distance from the end of the rod is formed at intervals of one-half period, alternatingly inverted and erect along the rod.
NSG America markets two distinct types of GRIN rod in diameters from 0.35 mm to 2.70 mm. One type is a relatively high numerical aperture rod which is generally cut to a length equal to or somewhat greater than one-quarter period so that an object at a moderate distance from one end is imaged on the other end. This type is called an Imaging SELFOC.RTM. Micro Lens (ISLW).
The other common type is of relatively low numerical aperture and long period length. It is called a SELFOC.RTM. Rod Lens (SRL) and is nominally cut to a length of one-half period or an integral number of periods up to as many as six or more in some diameters. NSG America also supplies these lenses assembled with or without end cover glasses and with or without thin stainless steel tubing outer coverings so that they can be directly assembled into larger, heavier tubing with a fiberoptic light guide to illuminate the field of view. With a suitable ocular, generally a compound microscope, these assemblies are often used in endoscopes or borescopes.
In some applications, a videosensor is directly coupled with gradient index endoscopes by suitable optics so that the object field may be viewed on a video monitor screen.
Although advantageous in many respects, endoscopes or borescopes employing strictly gradient index elements are not without limitations. One of the these is that they are useful for viewing only directly along the axis of the optical system. In many cases, it is more desirable to view to one side or at an oblique angle. In conventional endoscopes, this problem is solved with an angle-of-view prism, a prism that changes the direction of sight. Conventional endoscopes and borescopes are commonly available with the direction of view departing from straight ahead by oblique angles as large as 120 degrees. It is also usually desirable to have a field of view in air of a minimum of 40 degrees or more and, preferably, of 80 to 100 degrees. Additionally, the angle of view is reduced by about one-fourth by immersion in water.
In conventional endoscopes, both problems are usually solved by designs employing a suitable prism, with or without a field-widener lens preceding the prism. It is common to design such prisms with two internal reflections to maintain proper reversion of the field of view. Many such line-of-view prisms, particularly all of those in the very popular range of 20 to 60 degrees, have narrow apertures inherent in their design so that only a very restricted ray bundle will pass through them. This problem is solved in endoscopes with homogeneous optics by having a reversed retrofocus objective with an external pupil preceding the first element. This pupil, which is much smaller than the diameter of the relay optics, is positioned within the prism so that the narrow prism apertures do not vignette the field of view. If a further increase in the field of view is desired, a negative lens can then be placed on the distal face of the prism. With conventional gradient index endoscopes of previous practice, which consist of an objective section followed by a relay section, the combination is such that its entrance pupil is inside of the objective itself. Consequently, a line-of-view prism such as those used in conventional endoscopes with homogeneous optics severely vignettes the field of view.
One solution to the above problem is disclosed in U.S. Pat. No. 4,515,444 issued to Rochelle Prescott and Dennis C. Leiner on May 7, 1985 and entitled "OPTICAL SYSTEM". Here, use was made of a spacer of homogeneous optical material placed between two gradient index lenses, an objective and a relay. This system has the disadvantage of somewhat reducing the Lagrangian of the system below the value otherwise limited by the relay system. Inclusion of the spacer also adds an additional system component thus increasing complexity and cost.
A second solution is that disclosed in U.S. Pat. No. 4,735,491 issued to Susumu Takahashi on Apr. 5, 1988 and entitled "OPTICAL SYSTEMS FOR ENDOSCOPES". Here, however, the objective is of conventional homogeneous lenses, and the system therefore no longer has the advantages of the simplicity of construction possible with the solid monolithic structure of a gradient index system.
None of the known solutions have recognized or understood how to provide a gradient index endoscopic system that minimizes and even eliminates vignetting by the apertures of a preceding line-of-view prism or other preceding potentially vignetting aperture. It is therefore a primary object of the present invention to provide such a system.
It is another object of the present invention to provide an endomicroscope whose only change in optics takes place at the ocular or proximate end by refocussing or changing the optics.
It is yet another object of the present invention to provide an endomicroscope whose probe section, as distinguished from the ocular or a camera used with it, may be hermetically sealed and immersible in the usual disinfecting solutions of surgical practice, or, in borescope applications, sealed against the entry of dust and moisture.
It is still another object of the present invention to provide an endoscope having its first internal image formed at some location that is not a cemented or glass-air interface.
Still another object of the invention is to provide a method for designing such endoscopic systems.
Other objects of the invention will in part be obvious and in part appear hereinafter. The invention, accordingly, comprises the methods and apparatus possessing the construction, combination of elements, and arrangement of parts exemplified in the following detailed disclosure.